Conventionally, as the automatic focusing function (to be referred to as the AF function hereinafter) of a camera, a hill-climbing scheme has been proposed by, e.g., Japanese Patent Laid-Open No. 62-103616 and known. In recent years, as image sensing apparatuses used for video cameras, 3-CCD type apparatuses used by broadcasting stations or for operational use have been widely used. Accordingly, the image quality increasingly improves, and the focusing accuracy is also rapidly increasing. This tendency is expected to be strong in the future.
For lenses as well, along with intensive competition for values added and discriminations, the magnification becomes high, and the AF accuracy of a lens is required to be high.
However, in the conventional image sensing apparatus, the high image quality by the 3-CCD scheme and the high magnification of a lens are inconsistent from the viewpoint of improvement of they AF function. This is a very important subject of study.
Since a lens has chromatic aberration due to its optical characteristics, all color components do not always form images on the same plane. More specifically, in the 3-CCD scheme in which a light beam the CCDs have different focus positions. Especially, a zoom lens capable of changing its magnification can hardly focus all chromatic aberrations at the respective zoom positions onto the same plane and is impractical.
In recent years, a higher magnification is required. Conversely, as the magnification increases, the chromatic aberration tends to be enlarged. An enlarged chromatic aberration means that, for example, even when a green object is in focus, red and blue objects on the same plane separate from the best focus state due to the high magnification.
In some systems, an attachment lens is attached to the front portion of a lens, or the camera and lens are interchangeable. Abundant accessories including an extender or tube attached between the camera and the lens to convert the magnification or change the optical characteristics are also important as products. A lens portion including such an accessory attached to the lens is called a lens system. The accessory largely influences the chromatic aberration in the lens system.
Hence, for the conventional AF control, a high focus accuracy is required for a high image quality, while AF at the best focus position cannot be obtained because the chromatic aberration is enlarged due to the high magnification.